Why do so many of us feel tired all the time?, asks New Scientist. According to a recent survey of more than 20,000 people by researchers at Radboud University in the Netherlands, about 30% of visits to doctors involve complaints about constant tiredness, while one in five people in the US report having experienced fatigue intense enough to interfere with living a normal life.

It’s perhaps surprising, then, writes Emma Young, that we are only now beginning to work out what fatigue actually is. 'Until recently, daytime tiredness was presumed to be nothing more mysterious than simple physical exhaustion or feeling the need to sleep – the US Center for Disease Control and Prevention estimates that 35% of people are short on sleep. Combine that with the fact that tiredness is subjective and therefore difficult to measure, plus the subject falls somewhere between studies of the body and mind, and it’s small wonder fatigue has largely escaped scientific scrutiny'.

A handful of researchers are now trying to figure out the causes, says Young. Could it be that is life more exhausting than it has ever been because of the competing demands of work and family? Anna Katharina Schaffner, a historian at the University of Kent in Canterbury, who is quoted in Kelly’s article, thinks this is a fallacy. People through the ages have consistently complained of being worn out, and harked back to the relative calm of simpler times.

If modern life isn’t to blame, another possibility is that at least some fatigue is down to a lack of sleep. Researchers distinguish between the need for sleep and fatigue, however, considering them to be closely related but subtly different. So if it’s not the same thing as sleepiness, what is fatigue? Mary Harrington, a neuroscientist at Smith College in Northampton, Massachusetts, is one of a handful of researchers looking for a telltale biological signal of fatigue.

One possibility Harrington is investigating is that daytime fatigue stems from a problem with the circadian clock, which regulates periods of mental alertness through the day and night. Too little light in the mornings, or too much at night, can lead to a lethargic day. 'I think circadian rhythm disruption is quite common in our society and is getting worse with increased use of light at night', says Harrington.

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